In a design field of a dynamo-electric machine, which has a rotor including permanent magnets as magnetic poles of the rotor, it has been demanded to reduce the amount of a rare material (e.g., a rate earth element), which is used as a material of the permanent magnets. In order to satisfy the above demand, it is conceivable to implement one design method in such a manner that the magnetic poles of the rotor are formed by the permanent magnets, which are intermittently arranged in the circumferential direction. In this way, in comparison to a case where the magnetic poles of the rotor are formed by a permanent magnet, which is magnetized to have the magnetic poles and is configured into a ring form, the total amount of the material of the permanent magnets can be reduced by eliminating the material from an intermediate location between each circumferentially adjacent two of the permanent magnets.
Various shapes have been proposed as a shape of each permanent magnet used in the above design method. For example, in a cross section of the rotor, which is take in a direction perpendicular to a rotational axis of the rotor, each permanent magnet may be configured into an arcuate form that extends along an outer peripheral surface of a rotor core, which has a circular cross section. Alternatively, each permanent magnet may be configured into an arcuate form having two projections, which radially inwardly project toward the rotor core at two circumferential sides, respectively, of a circumferential center of the permanent magnet, as disclosed in JP3833256B2 (corresponding to U.S. Pat. No. 5,939,809A). Further alternatively, each permanent magnet may be configured to have a flat opposing surface, which is radially opposed to the rotor core and is entirely flat.
However, in the case of the permanent magnet, which is configured into the arcuate form, a radius of curvature of the opposing surface of the permanent magnet, which is radially opposed to the rotor core, i.e., a radius of curvature of the inner peripheral surface of the permanent magnet may not coincide with a radius of curvature of the outer peripheral surface of the rotor core in some of the products due to presence of a manufacturing tolerance. Furthermore, the radius of curvature of the inner peripheral surface of the permanent magnet, which is configured into the arcuate form, may be changed through expansion or contraction of the permanent magnet depending on a temperature change. Therefore, in some cases, depending on a quality of the manufactured permanent magnet or the expansion or contraction of the permanent magnet caused by the temperature change, a gap may be formed between the circumferential center portion of the permanent magnet and the rotor core while the two circumferential ends of the permanent magnet contact the rotor core. In such a case, when an external force is applied to the permanent magnet, a stress is concentrated in a center portion of the permanent magnet to possibly cause a damage (e.g., breakdown, cracking, chipping) of the permanent magnet.
In the case of the permanent magnet, which is disclosed in JP3833256B2 (corresponding to U.S. Pat. No. 5,939,809A) and is configured into the arcuate form having the two projections, two circumferential ends of the permanent magnet do not contact the rotor core. Therefore, it is possible to limit the deformation of the permanent magnet, which results in displacement of the circumferential center portion of the permanent magnet away from the rotor core while the two circumferential ends of the permanent magnet serve as a fulcrum at the time of occurrence of the expansion or contraction of the permanent magnet caused by the temperature change. However, in the case of the permanent magnet, which is disclosed in JP3833256B2 (corresponding to U.S. Pat. No. 5,939,809A), the two projections of the permanent magnet contact the rotor core, and the gap is formed between the circumferential center portion of the permanent magnet and the rotor core. Therefore, similar to the permanent magnet, which does not have the projections, when an external force is applied to the permanent magnet, a stress may be concentrated in a center portion of the permanent magnet to possibly cause the damage of the permanent magnet.
In the case of the permanent magnet having the flat opposing surface, which is radially opposed to the rotor core, a gap is not formed between the center portion of the permanent magnet and the rotor core regardless of the expansion or contraction of the permanent magnet caused by the temperature change. In this case, even when an external force is applied to the permanent magnet, a stress is dispersed in the circumferential direction and is thereby not concentrated in the center portion of the permanent magnet. However, even in the case of the permanent magnet, which has the flat opposing surface, a possibility of warping of the permanent magnet is increased when the surface area of the flat opposing surface of the permanent magnet is increased. When a cutting process is added to improve a surface accuracy of the contacting surface of the permanent magnet, manufacturing costs are increased, and a yield rate of the permanent magnet may possibly be deteriorated.